In the forests of central Africa, there’s a plant that looks like it’s growing its own Christmas decorations. Shiny baubles sprout from between its leaves, shimmering in a vibrant metallic blue. Look closer, and other colours emerge – pinpricks of red, orange, green and violet. It looks as if Seurat, or some other pointillist painter, had turned their hand to sculpture.

But these spheres, of course, are no man-made creations. They’re fruit. They are the shiniest fruits in the world. Actually, they are the shiniest living materials in the world, full-stop.

They belong to a plant called Pollia condensata, a tropical metre-tall herb that sprouts its shiny berry-like fruits in clusters up to 40-strong. These little orbs are iridescent – they use special layers of cells, arranged just so, to reflect colours with extraordinary intensity. This trick relies on the microscopic physical structures of the cells, rather than on any chemical pigments. Indeed, the fruits have no blue pigment at all.

In the animal kingdom, such tricks are commonplace – you can see them at work on the wings of a butterfly, the shells of jewel beetles, or the feathers of pigeons, starlings, birds or paradise and even some dinosaurs. But in the plant world, pigments dominate and structural colours were thought to be non-existent are much rarer.

Silvia Vignolini from the University of Cambridge discovered Pollia’s secret at Kew Gardens in the UK. Her group, led by Ullrich Steiner, was scouring the plant world for species that bend light in interesting ways. Under the recommendation of the Smithsonian’s Robert Faden, Vognolini sought out Pollia, and with help from Kew’s Paula Rudall, she found a sample of the plant. It was collected from Ghana in 1974 but it’s still as vivid as ever. (Unlike pigments, structural colours don’t degrade, so the fruits will retain their sheen for decades to come. Some fossils still keep their iridescence.)

Under the microscope, Vignolini saw that the outer part of the fruit consists of three to four layers of thick-walled cells (labelled “1” in the image below). Each cell contains yet more layers, made of cellulose fibres. The fibres all run parallel to one another, but each layer is slightly rotated against the one above it, producing an elegant spiral.

As light hits the top layer, some gets reflected and the rest passes through. The same thing happens at the next layer, and the next, and so on. Provided the layers are exactly the right distance apart, the reflected beams of light amplify each other to produce exceptionally strong colours. The technical term is “multilayer interference”. Or alternatively: “Ooh, shiny!”

Many animals use such structures to produce colour. This is why, like Pollia fruits, the wings of many butterflies and the feathers of many birds, can still look stunning after years in a fusty museum drawer. By finding the same structures in Pollia, Vignolini has uncovered a great example of convergent evolution, where species from different branches of the tree of life arrive at the same adaptations independently.

But Pollia fruits reflect more light than any bird or butterfly. Vignolini hasn’t just found the first strong iridescent colours in a plant; she’s found the strongest iridescent colours in nature. Or alternatively: “Ooh, SHINY!”

And the fruits have another unique trick. The distance between the cellulose layers varies from cell to cell. This means that the wavelength (and thus, the colour) of the light they reflect also vary from cell to cell. “Blue is the dominant one,” says Steiner, “but there’s the occasional red or green or yellow one. That’s why it looks pixellated.” Or pointillist, depending on whether you prefer your metaphors based on screens or canvas.

Why does Pollia have such bright fruit? Here’s a clue: you can’t eat them. Well, you can eat them, but there would be no point, because they provide next to no nourishment. They’re practically a dry seed-filled husk. Here’s another clue: Pollia grows in the same regions as another plant, Psychotria peduncularis, which also produces blue berries.

The team thinks that Pollia is mimicking the tasty blue fruits of its neighbour, tempting birds with the promise of tasty pulp, but rewarding them with nothing but seeds to carry. Alternatively, birds could collect the fruits to decorate their nests, or to use in mating displays. Either way, Pollia gets a free ride, and avoids having to spend energy on making sweet, nourishing tissues. It’s an evolutionary triumph of style over substance.

Comments (27)

Brian Too

That is strikingly beautiful!

However I’m not impressed by the idea that it is mimicking another blue fruit. There are many blue fruit of course, and none of them look anything like this.

Nor do I think I’m being anthropocentric. Even if you account for lower resolution eyes, limited colour sensitivity, compound eyes and all the rest. I do believe such a fruit would be noticeably different from almost anything else in the wild, to almost any visual system except the most primitive.

Perhaps the adaptive advantage at work here is merely to attract attention. Certainly it got ours!

Wouldn’t something that is bio luminescent be shinier? OK so maybe that doesn’t count. But what about a fish that resembles silver? But it is beautiful, and I especially love the description of the optical properties. Great post. Thanks.

@Professor dJinn – Good question, re: bioluminescence. Behold as I sort of answer it and sort of rely on sleight of hand.

The fruit has the greatest reflectivity of any known biological substance, including beetle shells, butterfly wings, and bird feathers. This covers the fish. Note: reflective. This means that the actual brightness depends on the amount of light shining on it in the first place. So, as long as the source of environmental light is bright enough (say, an afternoon, summer sun) it should, in theory, always be able to produce more than organisms that manufacture their own glow.

Ed, this is neat, but it is by no means the only plant which uses structural color. There is a whole world of iridescence out there. For example, the African iris Moraea aristata, Moraea villosa, and relatives have shiny iridescent blue patches on their petals. There are many beautiful images of them on the web. Other species with iridescence include many Begonia species with blue/green iridescence on their leaves (google Begonia pavonina for some amazing images), some aroids with iridescent leaves, some ferns with blue iridescence, and even some iridescent clubmosses (look up Selaginella willdenowii). Some understory tropical rainforest fruits from other genera also seem to show structural color, usually blue.
I am also a bit skeptical of the claim that these are the shiniest living thing, even though the original PNAS article does claim this. The authors don’t give details about which Morpho species was tested, and how; I can’t get the article they cite for the Morpho data. There are large differences between reflectiveness of the different Morpho species. Morpho amathonte, cypris, and sulkowskii are probably the most iridescent, while the more common M peliades and other similar species are orders of magnitude less reflective.

I love this article and photography, but I think some of the Adult Male Haplochomins (Cichlid fish from Lake Malawi Africa) in full breeding dress could give the fruit a run for the money in the area of shine (or any large scaled silver fish as was already mentioned in another comment).

Regardless of whether this is or is not the shiniest known living thing, I’m never (ever) happy about headlines which claim that something is “The most [adjective] [noun] in the world”. We can never know this. There’s always the possibility that a more [adjective] [noun] than the one we think we’re the first to discover will be found, or even has been found by someone else who didn’t publish it, or whose paper we don’t know about, or whatever.

I really wish such headlines could be avoided, since they always make a claim which goes beyond what we can know. “The most [adjective] [noun] known”, with the understanding that known means “known to the writer” or “published in major journals”, might be OK; but in the world or the world’s is in my opinion always dodgy.

I think it’s one of the things which encourages non-science journalists to treat tentative results as absolute fact and misrepresent the scientific process. It contributes to the false idea that science is about absolute knowledge. Just a tiny contribution; but still a contribution.

Thanks Ed for fixing that. About the other cases being “weak” iridescence, I recommend seeing for yourself by doing an image search in Google using the names of the plants I mentioned. Some of them are pretty dramatic.

I look forward to finding out what Morpho species was measured. The choice of species makes a big difference. Some of the species I mentioned can be seen from a plane flying over the jungle, flashing like signal mirrors. They are among the most amazing things I have ever seen. If this fruit is even more reflective, that will be very impressive!

Ed, I found a pdf of a 2002 Morpho article by Kinoshita et al. Kinoshita is also the lead author of the 2008 article that is cited by Vignolini et al in their claim that Pollia has a higher reflectance than Morpho wings. As luck would have it, the 2002 article tested two Morpho species, M didius and one of the brilliant species I mentioned above, M. sulkowskii (one of the most beautiful living things on the planet, which I sometimes see in my yard here in Ecuador). Morpho didius scales average about 30% reflectance (eyeballed from a graph) over the visible spectrum, with a peak reflectance of 55% in the blue part of the spectrum. The Pollia fruit is also reported to have 30% reflectance, so these two species are about tie for reflectance. But Morpho sulkowskii has a peak reflectance of 70% in the blue (!) and has a MINIMUM of 30% reflectance (at the red end of the spectrum). So its average reflectance over the range of visible wavelengths is far above 30%, the reported reflectance of Pollia. Maybe measurement methods are not comparable or maybe there is some other explanation, but I think the claim that Pollia is the shiniest living thing is wrong.

The harlequin glorybower is an ornamental tree that is native to China & Japan, but grows in large portions of the US. It has metallic blue berries just like this, but nestled in bright fuschia sepals.. They’re pretty spectacular!

@Tim J – Your point is well made, but I think it is implicit when we talk about records – and I really don’t think it’s a problem to talk about records – that we’re describing what is known. Because, as you point out, it would be patently illogical to extend the claim to what is unknown. The bit about misrepresenting science as a series of facts and not a process is very well taken, however.

“In that paper, they compare the reflectivity with diffusion standard not a silver mirror. Second, the method of measurement is different. They collect the total reflectivity with integration sphere, we measure local reflectivity at normal incidence. Reflectivity from flowers with integrating sphere can reach 80% of reflectivity, but that is due to scattering more than directional reflection, which is what we measure. We specify in our paper methods, using a Morpho, that the reflectivity with our setup was about 20%.”

Thanks Ed and Dr Vignolini. Dr Vignolini, maybe I misunderstand your last sentence, but I see no mention of your own Morpho measurements in your paper’s Methods section, so I am confused. Anyway, my main question was “Which Morpho species was compared to Pollia?” As I pointed out above, there are huge differences in reflectivity between species. Also, for the more reflective species, the angle of maximum reflection is very narrow and there is the potential to miss it.
Regards, Lou

It`s very american to label things as “the most” this or that even in scientific papers. European scientists usually don`t adopt this point of view in their researches. The “the most” thing is of course useful if one wishes to make science attractive to non-scientifics and especially children but it can only endanger scientific objectivity if used at the research level. Ethologically it would be interesting to know whether the bird populations used to pick up these vegetal beads are especially attracted by the shining as there are everywhere bird species which like to collect small shining objects.

The team thinks the plant is “mimicking the tasty blue fruits of a neighbor plant”.
Hmm, I’m having a tough time with mimicking because of the ‘awareness factor of mimicking?… requiring one plant to be ‘aware of a competing neighbor, in order to mimic it.

I prefer to observe the plant ‘as is’ and to realize that its beauty is obviously part of the mechanism for propagation, in that birds are attracted to shiny objects. But the plant is not ‘aware of the birds, any more than it is aware of the shiny neighbor plant, or us talking about it and wishing we had one in our homes…

There is no “awareness factor” in mimicry. Assuming Steiner is right, the plant will have gradually evolved an appearance that matches those of the blue berries around it, just like virtually every other mimic in the natural world.

Thanks Ed—my concern is mostly about the way the popular press like to make everything absolute (except global warming, which they like to pretend is still disputed), and trying to avoid giving them opportunities to do that.